Today, as light screens or shades for windows, for instance, blinds, curtains, ground glass, etc. are utilized. The dimming (or controlling) or screening means, typically blinds and curtains, are used to shield off or attenuate incident solar or other light or protect the privacy of a specific zone against visual access from the outdoor or other indoor zone by rolling down or otherwise bringing them into a barrier-wise effective position. Studies are in progress on methods for accomplishing such opening and closure in a power-aided manual mode or in an automatic mode for sophisticated control of dimming or light shielding. However, the mechanical system using a motor or the like as a power source has the drawback of unpleasant motor noise and, in addition, because of the increased scale of the equipment, the system can hardly be used universally.
Therefore, physicochemical means utilizing high-performance compound glass materials have been proposed. For example, a liquid crystal glass adapted for a modulated light transmission (transparency)/barrier (blinding) performance in response to an applied electric field has been developed. However, when a polarizer is used in combination with a liquid-crystal glass, the clarity parameter is considerably sacrificed by the polarizer to obscure the vision. On the other hand, when the polarizer is omitted, a haze is consistently omnipresent because a difference in refractive index as such is utilized.
Meanwhile, ground glass is frequently used in privacy-protecting applications, for example in the windows of a conference room or an office room. However, unlike a blind or a curtain, ground glass has no light-shielding or dimming function.
Japanese Patent Publication No. 23471/1995 (JP-B-7-23471) discloses a lyotropic liquid-crystal composition adapted to assume a cholesteric liquid-crystalline structure in the presence of a solvent which comprises a solution of a hydroxyethylcellulose in water, an organic solvent, or a mixture of water and an organic solvent.
Japanese Patent Application Laid-open No. 220453/1994 (JP-A-6-220453) discloses a liquid crystal composition comprising a lyotropic cholesteric liquid-crystal composed of a linear homopolysaccharide derivative, typically hydroxypropylcellulose, and water supplemented with 0.05 to 5 weight % of a water-soluble electrolyte and a laminate article or window produced by layering the composition on a substrate which is transparent at least in part. This literature states that the above liquid crystal composition undergoes a transition from a liquid crystal phase to a random cohesive or aggregate phase to develop opacity by scattering visible light, that addition of a water-soluble electrolyte thereto depresses the opacification temperature of the liquid crystal composition to room temperature or a temperature close to room temperature, and that an automatic switching from the transparent liquid-crystal phase to the opaque scattered phase or vice versa can be achieved with the direct solar radiation energy.
However, none of the above literatures refer to the possibility of dimming or shielding off light with an electric field. In JP-A-6-220453, in particular, the clarification and opacification of a solution are controlled by means of temperature and electrolyte concentration. However, weather changes independently of temperature so that when the proposed device is applied to a window, for instance, its opacification may result in a successful light-shielding or -screening effect in the hot summer months but no opacification may take place in the winter months, thus failing to discharge the expected light-shielding function. Moreover, there may be cases in which the window is opacified despite good weather to obscure the exterior landscape. Thus, it is difficult to let the device discharge its light-shielding function in a satisfactory way.